Intra-oral x-ray imaging using a solid-state x-ray sensor is known. Bitewing x-ray images are used for crowns of teeth, that is a portion of the teeth above a gum, for both upper and lower teeth in a portion of a mouth. Bitewing x-rays are used to examine a thick, dense part of a tooth, e.g. enamel and dentine, and are used to determine thickness of enamel and the presence of decay (caries) between teeth. A best image contrast could in principle be obtained using a high-energy portion of a dental x-ray spectrum, e.g. between 35 keV and 70 keV, but typically a broad spectrum between 10 keV and 70 keV is used.
Periapical x-rays images show an entire tooth, including a root and surrounding bone. The root fits into fibrous bone below the gum, and is usually twice as long as the crown. Periapical x-rays examine a thin, less dense portion of the tooth and the low-density fibrous bone surrounding the root. Periapical x-rays are used to examine for root fracture, bone loss due to gum disease, and condition of root canal and periodontal ligament (which supports the tooth in its bony socket). Since bone loss can be caused by gum disease, detail of the gum where it meets the tooth is important. Periapical x-rays are also used to aid diagnosis following presentation of symptoms of pressure sensitivity, e.g. diagnosis of an abscess or cyst in the fibrous bone at the base of the root. In all cases, periapical x-rays are used to examine low density tooth and bone and image detail may be of soft tissue. Best image contrast could in principle be obtained by taking advantage of photo-electric x-ray attenuation in soft tissue using a low-energy portion of the dental x-ray spectrum, e.g. between 10 keV and 35 keV, but typically a broad spectrum between 10 keV and 70 keV is used.
Because different photon energies are required to obtain high resolution, high contrast images for different portions of a tooth and the surrounding tissue, it is not normally possible to obtain a single high resolution, high contrast image of a whole tooth and the surrounding tissue with a single exposure.
In other known systems:                a second image is generated after a first image is read out, which for a large area sensor typical of intra-oral, mammo- or chest radiography, could take several seconds—registration of the two images will usually be adversely affected due to motion effects;        the two images may be generated from two different sensors and the spatial registration may be poor;        the two images may be generated via filtering in order to obtain high and low sensor channels, and such filtering can rarely achieve desirable characteristics such as sharp cut-off and discrimination of the pass-bands.        
U.S. Pat. No. 6,381,301 describes a system which uses two complete mechanical scans of an object at two different x-ray energies. The two scan images are then combined. There is likely to be poor registration of the two images since each of the scans is likely to take at least 10seconds and the object may move between the scans. The registration also depends on generator and detector positioning accuracy.
U.S. Pat. No. 6,285,740 describes a dual-energy x-ray system, in which the detector consists of a soft x-ray detector and a hard x-ray detector in tandem, with an inter-detector filter in between. Alternatively suitable high energy/low energy scintillators are arranged to produce light of different colours, which are then imaged by CCD detectors for the capture of separate images. The detector is likely to have poor selectivity of the two x-ray energies. The sensitivity for the higher x-ray energy is likely to be poor since the high energy photons have to pass through the low energy detector before being imaged. The registration of the two images is likely to be poor because two detectors are used, also the inter-detector filter may produce undesirable spatial offsets.
U.S. Pat. No. 5,841,833 describes a dual energy detector with high and low energy detecting elements each based on an x-ray sensitive scintillator and a photodiode, arranged alternately in at least two rows and with low and high energy detector elements also arranged in columns of alternating high and low energy detector elements perpendicular to the rows. There is interpolation of the signals from adjacent, e.g. high energy pixels, and the signals are combined to produce an interpolated value for a virtual detecting element of high energy. The system is based on a linear, not an area, array, and hence the object must be scanned. The arrangement of the detector pixels is described, but not how the adjacent low and high x-ray energy pixels are actually realised. There may, for instance, be poor discrimination of the two x-ray energies.
U.S. Pat. No. 6,683,934 describes first and second filters which are selectively switched so as to be disposed between a source and a space accommodating a body, independently and respectively, for first and second, different x-ray energy levels. The filters interposed can never be ideal and the system is likely to have poor discrimination of the two x-ray energies.
It is an object of the present invention at least to ameliorate the aforesaid shortcomings in the prior art.